The present invention relates to a musical tone producing device and, more particularly, to a musical tone producing device of waveform memory readout type.
A musical tone producing device of this type is used as a musical tone generator such as an electronic musical instrument. The musical tone producing device is used to generate a musical tone signal whose waveform (tone color) elaborately changes in the same manner as tone colors of natural sounds produced by conventional musical instruments. For this purpose, a musical tone producing device is proposed, wherein waveform data of sampling points of the entire musical tone signal waveform from the beginning of the conventional musical tone to its end are stored in a waveform memory, and the waveform data of the respective sampling points are sequentially read out from the waveform memory to produce a musical tone signal. Such an example is shown in FIG. 3 of U.S. Pat. No. 4,383,462. In FIG. 3, a complete waveform of the period from the beginning till the end of a tone production is memorized to be read out subsequently. The complete waveform is stored in the WM31 and the waveform is read out based on a signal (KD) indicative of a key depression timing.
According to the technique described above, when waveform data obtained by sampling a tone of a conventional musical instrument are stored in the waveform memory, the same tones as in the conventional musical instrument can be produced. However, the amount of data to be stored in the waveform memory becomes large, and a compact, low-cost musical tone producing device cannot be obtained. It is thus desirable to decrease the capacity of the waveform memory.
In order to solve the above problem, a method of decreasing the capacity of the waveform memory is also proposed, wherein waveform data corresponding to a repeating waveform (i.e., waveform portions of the entire waveform which are periodically repeated) is stored in the waveform memory and is read out repeatedly. Such an example is shown in FIG. 6 of U.S. Pat. No. 4,383,462. In the WM61 in said FIG. 6 is stored a complete waveform of the attack period and the attack waveform is read therefrom based on a key depression (a KD signal). After the reading out of the attack waveform (an IMF signal) until the finishing of a tone production (a DF signal), the musical tone waveform of the fundamental period is read out repeatedly.
According to this technique, however, since waveform data to be stored in the waveform memory are basically obtained by sampling the amplitude of the musical tone waveform, the musical tone generated from the musical tone producing device becomes artificial. As a result, monotonous expressions in musical performance cannot be avoided.
In addition to the above disadvantage, according to this technique, the waveform data normally requires a number of bits to represent a maximum amplitude of the waveform. As a result, the capacity of the waveform memory must be increased.
In order to solve the above problem, a method of decreasing the capacity of the waveform memory is described in U.S. Pat. No. 3,515,392, wherein differences between waveform amplitudes of every two adjacent sampling points of the tone signal waveform are sequentially calculated, and difference data are stored in the waveform memory. The number of bits which represents the difference data is then smaller than that which represents the maximum waveform amplitude, and the required number of bits represents only a change in waveform. Therefore, the capacity of the waveform memory can be decreased.
When this method of storing the difference data in the waveform memory is considered in detail, however, the waveform includes a moderate-slope waveform portion and an steep-slope waveform portion subjected to abrupt, complicated changes. The difference greatly changes from a small value (represented by a smaller number of bits) to a large value (represented by a larger number of bits) in the latter. Therefore, when the number of bits for the difference data is simply limited without consideration, the musical tone signal produced on the basis of the limited difference data presents an artificial tone, resulting in inconvenience.